The proposed research is designed to elucidate the Escherichia coli functions which are necessary for the propagation of Bacteriophage lambda, and to identify the phage gene products which interact with them. We are using this virus-host interaction system also in order to achieve a better understanding of certain aspects of E. Coli physiology. In order to achieve these goals we have developed the necessary techniques for the isolation of Escherichia coli mutants which block the propagation of bacteriophage lambda by interfering at some step of the phage growth cycle following adsorption and DNA injection. Emphasis will be placed on the isolation of bacterial mutations which, in addition to blocking bacteriophage growth, render the cell itself temperature sensitive for growth. The analysis of such mutations in E. coli will include the following: (a) characterization of their effect on both phage and bacterial growth, (b) mapping of the bacterial loci defined by the mutations, (c) isolation of amber mutations in the gene(s), (d) identification of the gene product(s) on SDS polyacrylamide gels, (e) purification of the wild-type bacterial gene product by use of in vitro systems (to detect its activity), and (f) isolation and characterization of bacteriophage lambda mutants which can overcome the block exerted by the mutant host. By means of these studies we hope to characterize many of the bacterial functions involved in bacteriophage lambda DNA replication, RNA transcription and morphogenesis. It is hoped that this system will serve as a model for understanding of virus-host interactions in higher organisms.